In communication systems based on e.g. Code Divisional Multiple Access (CDMA), power control is used to meet the desired quality of service targets. The power control may be implemented both in a user equipment (UE) to meet a downlink quality target and also in a base station (BS) to meet an uplink quality target. In wireless communication networks, the downlink (DL) is the transmission path from the base station to the user equipment, and the uplink (UL) is the transmission path from the user equipment to the base station. It is important that the power control is able to maintain the desired quality of service target despite varying radio conditions, which is often the case in wireless communication systems.
Most CDMA systems, such as cdma 2000, Wideband Code Division Multiple Access (WCDMA), and the extensions of WCDMA called High Speed Packet Access (HSPA) and Evolved HSPA (HSPA+) applies inner loop power control and outer loop power control. The inner loop power control, also called fast power control, runs every time slot, which is typically less than 1 ms (e.g. 0.67 ms in WCDMA). In WCDMA the inner loop power control runs in both uplink and downlink. The fast inner-loop power control adjusts the transmit power of the sender towards a specific Signal to Interference and noise Ratio (SIR) target at the receiver. The aim of the uplink and downlink inner loop power controls is to counter the effect of fast fading, while maintaining the desired SIR target. In the uplink the power control also compensates for the near-far problem, so that a signal received from users far out in a cell are not swamped out by stronger signals from users close to the base stations. During every slot the user equipment estimates the SIR on some known reference or pilot symbols and compares it with some SIR target corresponding to a given service, e.g. Block Error Rate (BLER), certain Bit Error Rate (BER) requirements and spreading factor used etc. In WCDMA, downlink SIR is measured on Dedicated Physical Control Channel (DPCCH), which comprises pilots bits and Transmit Power Control (TPC) bits. The TPC bits, which correspond to TPC commands, are also used for uplink power control and the pilot bits are primarily used for channel estimations. If the estimated SIR is less than the SIR target then the user equipment generates an UP command, otherwise it generates a DOWN command. In response the base station will increase (in case of UP) or decrease (in case of DOWN) its downlink transmit power. TPC commands for uplink power control are carried on Dedicated Physical Channel (DPCH) or Fractional DPCH (F-DPCH) from the base station to the user equipment. The user equipment will increase (in case of UP) or decrease (in case of DOWN) its uplink transmit power in response to the received TPC commands.
The aim of the outer loop power control is to adjust the SIR target value used by the inner loop power control as previously explained, while maintaining a certain link quality. The quality target (e.g. BLER of the data) is set by the network and is expected from the user equipment to consistently maintain this target to ensure the desired quality of service is met throughout the session. Due to the varying radio link conditions e.g. user mobility, fast fading etc, the mapping between the SIR target and BLER changes over time. This is a key point as it requires frequent adjustment of the SIR target to maintain the desired value of BLER. This mechanism of adjusting the SIR target is also referred to as outer loop power control, quality control or outer loop scheme.
The transmission of data over the air in a wireless communication system is performed by using a plurality of different physical channels, for example Dedicated Physical Control CHannel (DPCCH), Dedicated Physical Data CHannel (DPDCH), Enhanced Dedicated Physical Control CHannel (E-DPCCH) and Enhanced Dedicated Physical Data CHannel (E-DPDCH). The power consumptions of these are generally related to each other by power offsets, i.e. beta-values or gain factor relative the power level of the DPCCH.
Currently the Third Generation Partnership Project (3GPP) is evaluating the potential benefits of uplink transmit (Tx) diversity in the context of High-Speed Uplink Packet Access (HSUPA). An aim is to enhance uplink capacity and UE power consumption. With uplink transmit diversity UEs that are equipped with two or more transmit antennas are capable of utilizing all of them for uplink transmissions. This is achieved by multiplying a UE output signal s(t) with a set of complex pre-coding weights wi, where i=1 . . . N with N denoting the number of transmit antennas. The rationale behind uplink transmit diversity is to adapt the pre-coding weights so that user and network performance is maximized. Depending on UE implementation the antenna weights may be associated with different constraints.
Within 3GPP two classes of transmit diversity are considered:                Switched antenna transmit diversity, where the UE at any given time-instance transmits from one of its antennas only. Thus if wi≠0, wj=0 for all j≠i.        Beamforming where the UE at a given time-instance can transmit from more than one antenna simultaneously. By means of beamforming it is possible to shape an overall antenna beam in the direction of a target receiver.        
While switched antenna transmit diversity is possible for UE implementations with a single power amplifier (PA) the beam forming solutions may require one PA for each transmit antenna.
Switched antenna transmit diversity can be seen as a special case of beamforming where one of the antenna weights is 1 (i.e. switched on) and the antenna weight of any other antenna of the UE is 0 (i.e. switched off).
A fundamental idea behind uplink transmit diversity is to exploit variations in the effective channel to improve user and network performance. The term effective channel here incorporates effects of transmit antenna(s), transmit antenna weights, receiving antenna(s), as well as the wireless channel between transmitting and receiving antennas.
A commonly considered application scenario of beamforming is a UE with two PAs that typically transmits with equal power on two antennas. Beamforming algorithms proposed in 3GPP use phase differences between the antennas based on a code book, which means that the phase difference is chosen among a set of possible phases. One algorithm to find the best phase difference discussed in 3GPP is to evaluate the effect of changing phase by examining the received TPC bits. For example, if the phase difference is increased and the corresponding TPC bits indicate that power should be increased, this implies that the increased phase difference resulted in worse channel and consequently, the phase should be restored or decreased.
The gain from using beamforming originates from the diversity gain that is obtained by using two or more transmit antennas. The increased gain means that lower transmission power is needed to reach the desired SIR target. In an interference limited system, the lower transmission power results in lower interference between cells (inter-cell interference), which may lead to higher cell throughput. For a power limited UE, the diversity gain is more or less directly shown as a coverage gain.
Channel conditions can differ a lot between different transmit antennas. There are two major reasons for this: fast fading differences between the antennas, and fixed antenna gain imbalance between the antennas. For a high speed user the difference between the antenna channels vary very fast, but for a stationary lap-top the channel differences can be more or less constant (or very slow varying). It is also common that a primary antenna has a better antenna gain than a secondary antenna. In the current 3GPP evaluations of UL Tx diversity, a fixed imbalance between two transmit antennas of 0 or 4 dB and a random (per UE) imbalance with standard deviation of 2.25 dB are being studied. Hence, it is not uncommon or unrealistic that there is a large antenna gain imbalance between the antennas.
From the above description it is apparent that there are many factors that may need to be considered in order to achieve efficient power control both in the downlink and in the uplink.